The present invention relates to a process and apparatus for producing a gaseous oxygen product at a delivery pressure by rectifying air. More particularly, the present invention relates to such a process and apparatus in which liquid oxygen is pumped to the delivery pressure and then vaporized within a main heat exchanger. Even more particularly, the present invention relates to such a process and apparatus in which the gaseous oxygen product is produced with a low concentration of heavy impurities.
In cryogenic air separation plants that produce gaseous oxygen at a delivery pressure by vaporizing pumped liquid oxygen within a main heat exchanger, heavy impurities such as carbon dioxide and hydrocarbons can exceed their solubility limits in the liquid oxygen as it vaporizes. As a result, carbon dioxide contained within the liquid oxygen can solidify to plug heat exchange passageways within the main heat exchanger and hydrocarbons such as acetylene can come out of solution to present a safety hazard. This occurs because the heavy impurities such as carbon dioxide and hydrocarbons have a much lower vapor pressure than oxygen and as such, tend to concentrate in liquid oxygen being produced within the air separation plant. When the liquid oxygen is raised to a higher pressure by pumping and then vaporized by being heated within the main heat exchanger of the air separation plant, the resulting vaporization temperature increases the vapor pressures of the heavy impurities to a degree greater than the oxygen vapor pressure increase and hence, the heavy impurities vaporize sooner before the liquid oxygen is fully vaporized.
Heavy impurity concentrations can be maintained below their solubility limit during the vaporization process by pumping the liquid oxygen to a higher delivery pressure. However, as the delivery pressure increases, the compression of the air being cooled within the main heat exchanger must also increase to maintain a positive temperature difference within the main heat exchanger. It is generally uneconomical from an energy standpoint to deliver oxygen at a higher pressure than required just to prevent heavy impurities from exceeding their solubility limits.
As will be discussed, the present invention provides a process and apparatus for the separation of air to produce a gaseous oxygen product at a delivery pressure with a low level of heavy impurity concentration and without delivering the product at a higher than necessary delivery pressure.